Optical fiber communication systems transmit and receive electromagnetic radiation at relatively low levels. As a result, detection is accomplished in these systems by using photodetection devices which exhibt a high degree of sensitivity to the received radiation levels. High degrees of sensitivity are achieved by incorporating amplification features such as avalanche multiplication, transistor action or photon feedback into the electronic design of the device.
Photon feedback is an internal amplification process in which charge carriers are multiplied in a photodetection device having two distinct semiconductor regions of dissimilar energy bandgaps. See, for example, U.S. Pat. No. 3,891,993 issued to H. Beneking on June 24, 1975. Primary photons impinging on a semiconductor region having a narrow bandgap cause charge carriers, i.e., electron-hole pairs, to be formed. Under the force of an electric field, these charge carriers are swept into the semiconductor region having a wide bandgap and undergo radiative recombination. In turn, secondary photons created by recombination impinge on the narrow bandgap semiconductor region to produce more charge carriers thereby achieving current amplification.
Multiplication of charge carriers is affected by the number of secondary photons which are fed back to the narrow bandgap semiconductor region. On the average, only one-half of the photons produced by radiative recombination in the wide bandgap semiconductor region propagate in the direction of the narrow bandgap region. Thus, charge carrier multiplication and current gain are limited to be no greater than two in presently known photon feedback devices.
Although a gain of two may be considered sufficient in some applications, current gains greater than two are needed to increase the sensitivity of photodetection devices appreciably for optical fiber communication system applications.